Wordnet 3.0
ADJECTIVE (2)
1. 
 less than human or not worthy of a human being; 
- Example: "treated natives as subhuman"- Example: "a subhuman spectacle"- Example: "the subhuman primates"2. 
 unfit for human beings; 
- Example: "subhuman conditions of life"
WordNet (r) 3.0 (2006):
subhuman
    adj 1: less than human or not worthy of a human being; "treated
           natives as subhuman"; "a subhuman spectacle"; "the
           subhuman primates" [ant: superhuman]
    2: unfit for human beings; "subhuman conditions of life"
Moby Thesaurus II by Grady Ward, 1.0:
61 Moby Thesaurus words for "subhuman":
   Draconian, Tartarean, animal, animalian, animalic, animalistic,
   anthropophagous, atrocious, barbaric, barbarous, beastlike,
   beastly, bestial, bloodthirsty, bloody, bloody-minded, brutal,
   brutalized, brute, brutelike, brutish, cannibalistic, cruel,
   cruel-hearted, demoniac, demoniacal, devilish, diabolic, dumb,
   fell, feral, ferocious, fiendish, fiendlike, fierce, hellish,
   infernal, inhuman, inhumane, instinctive, instinctual, mindless,
   murderous, nonrational, ruthless, sadistic, sanguinary,
   sanguineous, satanic, savage, sharkish, slavering, truculent,
   unchristian, uncivilized, unhuman, vicious, wolfish, zoic, zooidal,
   zoologic